Characterization of (Th,U)O2 pellets made by advanced CAP process

Coated Agglomerate Pelletization (CAP) process is being developed by Bhabha Atomic Research Centre (BARC) for the fabrication of ThO2-UO2 mixed oxide fuel pellets. In order to improve the microstructures with better microhomogeneity, a study was made to modify the CAP process. The advanced CAP (A-CAP) process is similar to the CAP process except that the co-precipitated powder of mixed oxide, ThO2-30%UO2 or ThO2-50%UO2, is used for coating instead of U3O8 powder. The choice of ThO2-UO2 powders as the coating material is advantageous compared to U3O8, since the presence of large quantities of ThO2 in UO2 powder gives better self-shielding effect. In this paper, ThO2 containing 4%UO2 (% in weight) was prepared by the A-CAP process. Property measurements including microstructure and microhomogeneity were made by optical microscopy, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), etc. It was found that the pellets sintered in air at 1400 °C showed a duplex grain structure and those sintered in Ar-8%H2 at 1650 °C showed a very uniform grain structure with excellent microhomogeneity.